Repo hygiene only - no code change by this task.
scripts/verify_import_closure.py (512 lines, py_compile OK) was already present but
untracked in the apps/pbls working tree when the Q4 task started. It is the companion
verifier for deploy_test round 3 question Q6 (28-module import closure: IMPORT_OK 15/28),
i.e. it belongs to a different task, not to Q4 (build.sh gate 6.4).
It was deliberately kept OUT of the Q4 commit bc4aaa2 (selective git add, never
git add -A) so the Q4 change set stays exactly build.sh + scripts/ddl_gate_check.py +
scripts/ddl/pbls_tables.sql.
Committed separately here because leaving a dirty working tree blocks the develop-stage
git closure gate. Content is untouched; authorship of the tool remains with the Q6 work.
